Proceedings of the 1999 Particle Accelerator Conference, New York, 1999
A COMPARISON BETWEEN PULSE COMPRESSION OPTIONS FOR NLC* S. G. Tantawi#$, R. D. Ruth, P. B. Wilson, SLAC, Stanford, CA Abstract We present a comparison, between options for pulse compression systems that provide rf power to the main linac of the Next Linear Collider (NLC). The parameters which are compared are efficiency, number of components and length of rf storage lines. Based on these parameters we produce a cost model for each system as a function of compression ratio, number of rf sources per unit system, and storage line parameters. The systems considered are Delay Line Distribution Systems (DLDS), Binary Pulse Compression (BPC), and Resonant Delay Lines (SLED-II). For all these systems we consider possible improvements through the use of several modes, active switches, and circulators.
1 INTRODUCTION During the past few years high power rf pulse compression systems have developed considerably. These systems provide a method for enhancing the peak power capability of high power rf sources while matching the long pulse of that source to the shorter filling time of the accelerator structure. In particular, future linear colliders, such as the proposed NLC[1] require peak rf powers that cannot be generated by current state-of-the-art microwave tubes. The SLED pulse compression system [2] was implemented to increase the gradient of the two-mile linac at the Stanford Linear Accelerator Center (SLAC). One drawback of SLED is that it produces an exponentially decaying pulse. To produce a flat pulse and to improve the efficiency, the Binary Pulse Compression (BPC) system [3] was invented. The BPC system has the advantage of 100% intrinsic efficiency and a flat output pulse. Also, if one accepts some efficiency degradation, it can be driven by a single power source [4]. However, The implementation of the BPC [5] requires a large assembly of over-moded waveguides, making it expensive and extremely large in size. The SLED II pulse compression system is a variation of SLED that gives a flat output pulse [6]. The SLED II intrinsic efficiency is better than SLED, but not as good as BPC. However, from the compactness point of view, SLED II is far superior to BPC. Several attempts have been made to improve its efficiency by turning it into a system using active switching [7]. However, the intrinsic efficiency of the __________________________________________________ *
This work is supported by Department of Energy Contract DE-AC0376SF00515. # Email:
[email protected] $ Also with the Communications and Electronics Department, Cairo University, Giza, Egypt.
0-7803-5573-3/99/$10.00@1999 IEEE.
active SLED-II system is still lower than that of the BPC. The DLDS [8] is a similar system to BPC, but by sending the rf upstream twords the gun it utilzes the return delay of the electron beam to reduce the length of the over-moded waveguide assembly. However it still uses more overmoded waveguide than that required by SLED-II. To further enhance the DLDS a variation on that system, the Multi-moded DLDS (MDLDS) [9] was introduced, which further reduces the length of the waveguide system by multiplexing several low-loss rf modes in the same waveguide. The system has an intrinsic efficiency of 100%, and the total over-moded waveguide length has been reduced considerably. We present a comparison, based on cost, for all various compression schemes that are available for the Next Linear Collider. In this comparison we do not describe the systems involved at any level of detail. The reader is referred to the cited references for details. However, it is our purpose to give an accurate formulation for the system efficiency, number of components, and length of delay lines or storage lines for each of these schemes as a function of compression ratio (the ratio of the source rf pulse width to the compressed pulse width). This will provide the basis for cost comparison. We will also extrapolate on the potential to expand and/or improve systems through the usage of a. Multi-moded structures, b. Active switching, c. Circulators.
2 COST MODEL Basically, the rf sources available now or in the near future will produce a pulse T of about 1.5 µs. The NLC accelerator structure needs a pulse of 380 ns. Hence, a pulse compression system which compresses the source rf pulse by a factor Cr=4 is required. The factor of 4 is the minimum required compression ratio. If rf sources can be improved to provide longer pulse lengths at the same peak power, one might utilize a bigger ratio. To achieve this pulse compression a storage system is employed to store the rf power until it is needed. Different portions of the rf pulse T are stored for different amount of times. The initial portion of the rf pulse is stored for a time period tm, the maximum amount of storage time for any part of T. The maximum value for tm is (1) tmmax = τ (Cr − 1) where τ = T is the accelerator structure pulse width, Cr and Cr is the compression ratio. The value given in by Eq.(1) is typical for most systems with the exception of
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Proceedings of the 1999 Particle Accelerator Conference, New York, 1999 DLDS, which stores the energy for only half the time. The realization of the storage system is usually achieved using low-loss waveguide delay lines. These lines are usually guides that propagate the rf signal at nearly the speed of light. The maximum length required for these guides, per compression system, is
l max = t m v g where
Cr , 2
(2)
v g is the group velocity of the wave in the delay
line. The total number of rf pulse compression systems required for the accelerator system is given by N aPa , (3) Nc = Pk nk C rηc where N a is the total number of accelerator structure in
C 1 Na Pa 1 − kms kms , (7) Am0 S m = Nk Am0 1 + kms r − 1 = + Cr 0 Cr0 ηc Pk Cr
where Am 0 is the cost of the modulator per klystron at a compression ratio
Cr 0 .
The cost of the rf pulse compression and power transmission is divided into two parts: a part that is dependent on the storage line length L and diameter D, and a part that is dependent on the number of components per pulse compression system nc. The storage line cost is divided into two parts: the cost of the vacuum system, which is a very weak function of diameter, and the cost of the pipes, which is directly proportional to the diameter. The cost model is, then, given by S c = (Alv + Alp D)L + Nc Acnc =
N aPa (Cr − 1) 1 τvg Rl (Alv + Alp D) + Ac nc nk Pkηc 2 Cr
(8)
the linac, Pk is the klystron (or the rf power source) peak power, Pa is the accelerator structure required peak
where
power, nk is the number of klystrons combined in one
Alp is the cost of waveguide pipe per unit length and
pulse compression system, and η c is the efficiency of the pulse compression system. Thus the total length of waveguide storage line for the entire linac is given by L = l max N c Rl =
1 N aPa t m v g . Rl n kη c Pk 2
(4)
C 1 N a Pa Ak 0 r Crηc Pk Cr 0
ak
,
(6)
Ak 0 is the cost per klystron at a compression ratio C r 0 . The exponent a k is a number that depends on the
where
details of manufacturing klystrons. For our present discussion we will assume that this number is 0.4 and C r 0 =4 [10]. The cost of conventional modulators is also dependent on the compression ratio. If the klystron pulse width is increased the stored energy in modulator is increased and hence its cost. A fraction
k ms of the modulator cost is due
to its energy storage elements [11]. The rest of the fractional cost,
diameter, and
Ac is the cost per component. The total
cost of the rf system normalized to the cost of one klystron Ak 0 is given by Sr (Cr , D, nk , Rl ; km , klp , klv , kc , kms ) =
where Rl is a length reduction factor which varies from one system to another and in general is also a function of the compression ratio. The total number of klystrons in the system is given by, 1 N a Pa . (5) Nk = Crηc Pk The cost of the rf system is divided into three different parts: cost of the klystron tube, the klystron power source and modulator, and the rf pulse compression and transportation system. The cost of the klystron tubes is given by S k = N k Ak =
Alv is the cost of vacuum system per unit length,
1 − k ms , is due to the rest of the system, in
particular the switching elements. Hence, a suitable model for the modulator cost is
ak 1 − kms kms kc Pa 1 Cr + + km + Pkηc Cr Cr0 Cr0 nk Cr
v
The parameters k m , k l factors and are given by km =
(Cr − 1) n Rl (klv + klp D) + c . Cr 2
(9)
, k c , k lp are the normalized cost
Alvτv g Am 0 A , k c = c , k lv = Ak 0 Ak 0 Ac
, and
k lp =
Alpτv g cm-1 (10) Ac
The cost of the modulators and components are normalized to the cost of a klystron at a compression ratio C r 0 . However, the cost of the delay line is normalized to the cost of a component. This is done because of the nature of the information available for the cost estimates at this time [12]. In the following we will compare all available pulse compression techniques based on the above-described criteria. To make the comparison more specific for the proposed NLC design we will make the following assumptions: a. The operating frequency of the system is 11.424 GHz b. The duration of the accelerator rf pulse is 380 ns. c. The basic waveguide delay system uses the TE01 mode. d. The next higher order modes that can be used are the two polarizations of the TE 12, and the two polarizations of the TE22 mode. Hence in calculating the efficiency of the compression system, the theoretical attenuation as a function of diameter for these modes is considered. e. The efficiency of transmission from the klystrons to the pulse compression system and from the pulse
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Proceedings of the 1999 Particle Accelerator Conference, New York, 1999 compression system to the accelerator structure is about 90%. Hence the total efficiency of the rf system is, ηc = 0.9η tηi ; where ηi is the intrinsic efficiency of the system, and η t is the efficiency of the delay lines. The maximum possible value is achieved with the DLDS using the TE01 mode in all waveguides. The number of accelerator structures N a is 9936
f.
3 CONCLUSION For most systems there is a considerable cost reduction as one goes to higher compression ratios. This implies the need for rf power sources that are capable of longer pulse width. The development of high power circulators, multimoded technology and active super-high-power switches will result in a considerable cost reduction for the NLC
for 1 TeV collider, and the power needed per accelerator structure Pa is 170 MW.
4 REFERENCES
[1] The NLC Design Group “Zeroth-Order Design Report g. Based on [12], we will assume that the ratios for the Next Linear Collider,” SLAC-Report-474, May -1 1996. k c = 0.012 , k m = 0.5 , k lv = 36 , k lp = 1.4 cm , [2] Z. D. Farkas et. al., "SLED: A Method of Doubling SLAC's Energy," Proc. of the 9th Int Conf. on High k ms = 1 / 3 , C r 0 = 4 Energy Accelerators, 1976, p. 576. h. The maximum amount of energy per rf pulse [3] Z. D. Farkas, "Binary Peak Power Multiplier and its Application to Linear Accelerator Design," IEEE E max that can be handled by rf components limits Trans. MTT-34, 1986. the maximum number of klystrons, nk , that can be [4] P. E. Latham, "The Use of a Single source to Drive a Binary Peak Power Multiplier," Linear Accelerator combined to provide power to a single pulse Conference, Williamsburg, Virginia, 1988, CEBAF-Rmax compression system; i.e., n k ≤ E /( Pk C rτ ) . 89-001, pp. 623-624. [5] Z. D. Farkas, et . al., "Two-Klystron Binary Pulse Compression at SLAC," Proc. of the IEEE Particle Single M oded DLD S Accelerator Conference, Wasington DC, May 1993, p. M ult i-M oded D LD S (number of modes=3) A ctive D LD S n =8 1208. k M ult i-M oded BPC (A high p ow er circulat or and 3 modes) M ult i-M oded SLED II (A high p ow er circulator and 3 modes) [6] P. B. Wilson, Z. D. Farkas, and R. D. Ruth, "SLED II: A ctive SLED II (One t ime Sw itching [7]) A New Method of RF Pulse Compression," Linear Multi-Moded D LD S (n =4, number of modes =3) Accl. Conf., Albuquerque, NM, September 1990; Single-Moded DLD S (n =4) SLAC-PUB-5330. 1.2 [7] S.G.Tantawi, et al. "Active High-Power RF Pulse Compression Using Optically Switched Resonant 1.1 Delay lines" IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES 1 IEEE, Aug. 1997. vol.45, no.8, pt.2, p. 1486-92 0.9 [8] H. Mizuno, Y. Otake, “A New Rf Power Distribution System For X Band Linac Equivalent To An Rf Pulse 0.8 Compression Scheme Of Factor 2**N,” 17th 0.7 International Linac Conference (LINAC94), Tsukuba, Japan, Aug 21 - 26, 1994, KEK-PREPRINT-94-112, 0.6 Oct 1994. 3pp. 0.5 [9] S.G. Tantawi, et al. " A Multi-Moded RF Delay Line 4 6 8 10 12 14 16 Distribution System for the Next Linear Collider,” to Compression Ratio be published in the proc. of the Eighth Workshop on Fig. 1 Comparison between different pulse compression Advanced Accelerator Concepts, Baltimore, MD, USA schemes. The relative cost is normalized to the cost of a 6-11 Jul 1998. single moded DLDS at a compression ratio of 4. [10] Robert M. Phillips, private communications. [11] Richard Cassel, private communications. i. Because components vary in their complexity we [12] Michael L. Neubauer, private communications. will give different weights to them. Hence in counting the number of components n c for each k
Relative Cost
k
system, we will assume that some complex components are the equivalent of several simple components. Fig. 1 shows the relative cost for several systems vs compression ratio. For each of these systems we calculated a general expression for the number of components, for the length reduction factor, and for the efficiency as a function of compression ratio.
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